[libc] Initial support for parsing SPIR-V for GPU libc (#128570)

Summary:
Some day we'd like to support this, add some initial support for parsing
the triple so it will at least attempt to build.

libc/cmake/modules/LLVMLibCArchitectures.cmake

diff --git a/libc/cmake/modules/LLVMLibCArchitectures.cmake b/libc/cmake/modules/LLVMLibCArchitectures.cmake
index fbb1091..b94534b 100644
--- a/libc/cmake/modules/LLVMLibCArchitectures.cmake
+++ b/libc/cmake/modules/LLVMLibCArchitectures.cmake
@@ -53,6 +53,8 @@
     set(target_arch "amdgpu")
   elseif(target_arch MATCHES "^nvptx64")
     set(target_arch "nvptx")
+  elseif(target_arch MATCHES "^spirv64")
+    set(target_arch "spirv64")
   else()
     return()
   endif()
@@ -162,6 +164,8 @@
   set(LIBC_TARGET_ARCHITECTURE_IS_AMDGPU TRUE)
 elseif(LIBC_TARGET_ARCHITECTURE STREQUAL "nvptx")
   set(LIBC_TARGET_ARCHITECTURE_IS_NVPTX TRUE)
+elseif(LIBC_TARGET_ARCHITECTURE STREQUAL "spirv64")
+  set(LIBC_TARGET_ARCHITECTURE_IS_SPIRV TRUE)
 else()
   message(FATAL_ERROR
           "Unsupported libc target architecture ${LIBC_TARGET_ARCHITECTURE}")