arch/xtensa: Support __thread and thread_local keywords

Enable architecture-specific support for those keywords. This is a
"gcc-based" thread local storage implementation.

boards/xtensa/esp32/common/scripts/simple_boot_sections.ld

@@ -403,6 +403,7 @@ SECTIONS
 
   .flash.rodata :
   {
+    _rodata_reserved_start = ABSOLUTE(.);
     . = ALIGN(4);
     _srodata = ABSOLUTE(.);
     *(EXCLUDE_FILE (*libarch.a:esp32_spiflash.* *libarch.a:esp32_spicache.*
@@ -469,8 +470,24 @@ SECTIONS
     *(.lit4.*)
     *(.gnu.linkonce.lit4.*)
     _lit4_end = ABSOLUTE(.);
+
+    /* TLS data. */
+
     . = ALIGN(4);
+    _thread_local_start = ABSOLUTE(.);
+
+    _stdata = ABSOLUTE(.);
+    *(.tdata .tdata.* .gnu.linkonce.td.*);
+    _etdata = ABSOLUTE(.);
+    _stbss = ABSOLUTE(.);
+    *(.tbss .tbss.* .gnu.linkonce.tb.* .tcommon);
+    _etbss = ABSOLUTE(.);
+
+    _thread_local_end = ABSOLUTE(.);
+
+    _rodata_reserved_end = ABSOLUTE(.);
   } >default_rodata_seg AT>ROM
+  _rodata_reserved_align = ALIGNOF(.flash.rodata);
 
   _image_irom_vma = ADDR(.flash.text);
   _image_irom_lma = LOADADDR(.flash.text);

boards/xtensa/esp32/esp32-devkitc/configs/ostest/defconfig

@@ -38,6 +38,7 @@ CONFIG_PREALLOC_TIMERS=4
 CONFIG_RAM_SIZE=114688
 CONFIG_RAM_START=0x20000000
 CONFIG_RR_INTERVAL=200
+CONFIG_SCHED_THREAD_LOCAL=y
 CONFIG_SCHED_WAITPID=y
 CONFIG_SPI=y
 CONFIG_START_DAY=6