[Support] Simplify macro conditions involving __GNUC__ (NFC)

We require GCC 7.4 these days, so we just need to check for
defined(__GNUC__).

Author: kazutakahirata

llvm/lib/Support/BLAKE3/blake3_neon.c

@@ -54,7 +54,7 @@ INLINE uint32x4_t rot8_128(uint32x4_t x) {
   // return vorrq_u32(vshrq_n_u32(x, 8), vshlq_n_u32(x, 32 - 8));
 #if defined(__clang__)
   return vreinterpretq_u32_u8(__builtin_shufflevector(vreinterpretq_u8_u32(x), vreinterpretq_u8_u32(x), 1,2,3,0,5,6,7,4,9,10,11,8,13,14,15,12));
-#elif __GNUC__ * 10000 + __GNUC_MINOR__ * 100 >=40700
+#elif defined(__GNUC__)
   static const uint8x16_t r8 = {1,2,3,0,5,6,7,4,9,10,11,8,13,14,15,12};
   return vreinterpretq_u32_u8(__builtin_shuffle(vreinterpretq_u8_u32(x), vreinterpretq_u8_u32(x), r8));
 #else 

llvm/lib/Support/ConvertUTF.cpp

@@ -79,7 +79,7 @@
 #  define ConvertUTF_RESTORE_WARNINGS \
     _Pragma("clang diagnostic pop")
 # endif
-#elif defined(__GNUC__) && __GNUC__ > 6
+#elif defined(__GNUC__)
 # define ConvertUTF_DISABLE_WARNINGS \
    _Pragma("GCC diagnostic push")    \
    _Pragma("GCC diagnostic ignored \"-Wimplicit-fallthrough\"")

llvm/unittests/Support/AlignOfTest.cpp

@@ -30,7 +30,7 @@ namespace {
 #ifdef __clang__
 #pragma clang diagnostic ignored "-Wunknown-pragmas"
 #pragma clang diagnostic ignored "-Winaccessible-base"
-#elif ((__GNUC__ * 100) + __GNUC_MINOR__) >= 402
+#elif defined(__GNUC__)
 // Pragma based warning suppression was introduced in GGC 4.2.  Additionally
 // this warning is "enabled by default".  The warning still appears if -Wall is
 // suppressed.  Apparently GCC suppresses it when -w is specifed, which is odd.