[libc][cmake] Do not overwrite SKIP_FLAG_EXPANSION_*. (#125762)

So that users can set these manually if needed.

Author: lntue

libc/cmake/modules/LLVMLibCFlagRules.cmake

@@ -268,23 +268,29 @@ set(EXPLICIT_SIMD_OPT_FLAG "EXPLICIT_SIMD_OPT")
 set(MISC_MATH_BASIC_OPS_OPT_FLAG "MISC_MATH_BASIC_OPS_OPT")
 
 # Skip FMA_OPT flag for targets that don't support fma.
-if(NOT((LIBC_TARGET_ARCHITECTURE_IS_X86_64 AND (LIBC_CPU_FEATURES MATCHES "FMA")) OR
-       LIBC_TARGET_ARCHITECTURE_IS_RISCV64))
-  set(SKIP_FLAG_EXPANSION_FMA_OPT TRUE)
+if(NOT DEFINED SKIP_FLAG_EXPANSION_FMA_OPT)
+  if(NOT((LIBC_TARGET_ARCHITECTURE_IS_X86_64 AND (LIBC_CPU_FEATURES MATCHES "FMA")) OR
+        LIBC_TARGET_ARCHITECTURE_IS_RISCV64))
+    set(SKIP_FLAG_EXPANSION_FMA_OPT TRUE)
+  endif()
 endif()
 
 # Skip EXPLICIT_SIMD_OPT flag for targets that don't support SSE2.
 # Note: one may want to revisit it if they want to control other explicit SIMD
-if(NOT(LIBC_TARGET_ARCHITECTURE_IS_X86_64 AND (LIBC_CPU_FEATURES MATCHES "SSE2")))
-  set(SKIP_FLAG_EXPANSION_EXPLICIT_SIMD_OPT TRUE)
+if(NOT DEFINED SKIP_FLAG_EXPANSION_EXPLICIT_SIMD_OPT)
+  if(NOT(LIBC_TARGET_ARCHITECTURE_IS_X86_64 AND (LIBC_CPU_FEATURES MATCHES "SSE2")))
+    set(SKIP_FLAG_EXPANSION_EXPLICIT_SIMD_OPT TRUE)
+  endif()
 endif()
 
 # Skip ROUND_OPT flag for targets that don't support rounding instructions. On
 # x86, these are SSE4.1 instructions, but we already had code to check for
 # SSE4.2 support.
-if(NOT((LIBC_TARGET_ARCHITECTURE_IS_X86_64 AND (LIBC_CPU_FEATURES MATCHES "SSE4_2")) OR
-       LIBC_TARGET_ARCHITECTURE_IS_AARCH64 OR LIBC_TARGET_OS_IS_GPU))
-  set(SKIP_FLAG_EXPANSION_ROUND_OPT TRUE)
+if(NOT DEFINED SKIP_FLAG_EXPANSION_ROUND_OPT)
+  if(NOT((LIBC_TARGET_ARCHITECTURE_IS_X86_64 AND (LIBC_CPU_FEATURES MATCHES "SSE4_2")) OR
+        LIBC_TARGET_ARCHITECTURE_IS_AARCH64 OR LIBC_TARGET_OS_IS_GPU))
+    set(SKIP_FLAG_EXPANSION_ROUND_OPT TRUE)
+  endif()
 endif()
 
 # Choose whether time_t is 32- or 64-bit, based on target architecture