[fuzz] Fix compiler detection & update ubsan flags

* Fix compiler version regex, which was broken for multi-digit
  versions.
* Fix compiler detection for gcc.
* Disable `pointer-overflow` instead of `integer-overflow` for gcc
  versions newer than 8.0.0.

tests/fuzz/fuzz.py

@@ -183,21 +183,21 @@ def compiler_version(cc, cxx):
     if b'clang' in cc_version_bytes:
         assert(b'clang' in cxx_version_bytes)
         compiler = 'clang'
-    elif b'gcc' in cc_version_bytes:
+    elif b'gcc' in cc_version_bytes or b'GCC' in cc_version_bytes:
         assert(b'gcc' in cxx_version_bytes or b'g++' in cxx_version_bytes)
         compiler = 'gcc'
     if compiler is not None:
-        version_regex = b'([0-9])+\.([0-9])+\.([0-9])+'
+        version_regex = b'\s([0-9]+)\.([0-9]+)\.([0-9]+)\s'
         version_match = re.search(version_regex, cc_version_bytes)
         version = tuple(int(version_match.group(i)) for i in range(1, 4))
     return compiler, version
 
 
 def overflow_ubsan_flags(cc, cxx):
     compiler, version = compiler_version(cc, cxx)
-    if compiler == 'gcc':
+    if compiler == 'gcc' and version < (8, 0, 0):
         return ['-fno-sanitize=signed-integer-overflow']
-    if compiler == 'clang' and version >= (5, 0, 0):
+    if compiler == 'gcc' or (compiler == 'clang' and version >= (5, 0, 0)):
         return ['-fno-sanitize=pointer-overflow']
     return []