Revert "arch: deprecate _current"

Mostly a revert of commit b1def7145f ("arch: deprecate `_current`").

This commit was part of PR #80716 whose initial purpose was about providing
an architecture specific optimization for _current. The actual deprecation
was sneaked in later on without proper discussion.

The Zephyr core always used _current before and that was fine. It is quite
prevalent as well and the alternative is proving rather verbose.
Furthermore, as a concept, the "current thread" is not something that is
necessarily architecture specific. Therefore the primary abstraction
should not carry the arch_ prefix.

Hence this revert.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

soc/espressif/esp32/soc.c

@@ -69,7 +69,7 @@ void IRAM_ATTR __esp_platform_start(void)
 	__asm__ __volatile__ ("wsr %0, PS" : : "r"(PS_INTLEVEL(XCHAL_EXCM_LEVEL) | PS_UM | PS_WOE));
 
 	/* Initialize the architecture CPU pointer.  Some of the
-	 * initialization code wants a valid arch_current_thread() before
+	 * initialization code wants a valid _current before
 	 * z_prep_c() is invoked.
 	 */
 	__asm__ __volatile__("wsr.MISC0 %0; rsync" : : "r"(&_kernel.cpus[0]));

soc/espressif/esp32/soc_appcpu.c

@@ -80,7 +80,7 @@ void IRAM_ATTR __appcpu_start(void)
 		: "r"(PS_INTLEVEL(XCHAL_EXCM_LEVEL) | PS_UM | PS_WOE));
 
 	/* Initialize the architecture CPU pointer.  Some of the
-	 * initialization code wants a valid arch_current_thread() before
+	 * initialization code wants a valid _current before
 	 * z_prep_c() is invoked.
 	 */
 	__asm__ __volatile__("wsr.MISC0 %0; rsync" : : "r"(&_kernel.cpus[1]));

soc/espressif/esp32s2/soc.c

@@ -62,7 +62,7 @@ void __attribute__((section(".iram1"))) __esp_platform_start(void)
 	__asm__ __volatile__("wsr %0, PS" : : "r"(PS_INTLEVEL(XCHAL_EXCM_LEVEL) | PS_UM | PS_WOE));
 
 	/* Initialize the architecture CPU pointer.  Some of the
-	 * initialization code wants a valid arch_current_thread() before
+	 * initialization code wants a valid _current before
 	 * arch_kernel_init() is invoked.
 	 */
 	__asm__ __volatile__("wsr.MISC0 %0; rsync" : : "r"(&_kernel.cpus[0]));

soc/espressif/esp32s3/soc.c

@@ -97,7 +97,7 @@ void IRAM_ATTR __esp_platform_start(void)
 	__asm__ __volatile__("wsr %0, PS" : : "r"(PS_INTLEVEL(XCHAL_EXCM_LEVEL) | PS_UM | PS_WOE));
 
 	/* Initialize the architecture CPU pointer.  Some of the
-	 * initialization code wants a valid arch_current_thread() before
+	 * initialization code wants a valid _current before
 	 * arch_kernel_init() is invoked.
 	 */
 	__asm__ __volatile__("wsr.MISC0 %0; rsync" : : "r"(&_kernel.cpus[0]));

soc/espressif/esp32s3/soc_appcpu.c

@@ -65,7 +65,7 @@ void IRAM_ATTR __appcpu_start(void)
 	__asm__ __volatile__("wsr %0, PS" : : "r"(PS_INTLEVEL(XCHAL_EXCM_LEVEL) | PS_UM | PS_WOE));
 
 	/* Initialize the architecture CPU pointer.  Some of the
-	 * initialization code wants a valid arch_current_thread() before
+	 * initialization code wants a valid _current before
 	 * arch_kernel_init() is invoked.
 	 */
 	__asm__ __volatile__("wsr.MISC0 %0; rsync" : : "r"(&_kernel.cpus[1]));