Drop enable format string warnings to help gcc9

Fixes
| /mnt/a/yoe/build/tmp/work/core2-64-yoe-linux-musl/mozjs/52.9.1-r0/mozjs-52.9.1/js/src/jit/x64/BaseAssembler-x64.h:596:13: error: '%s' directive argument is null [-Werror=format-overflow=]
|   596 |         spew("movq       " MEM_obs ", %s", ADDR_obs(offset, base, index, scale), GPReg64Name(dst));
|       |         ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Upstream-Status: Inappropriate [Workaround for gcc9]
Signed-off-by: Khem Raj <raj.khem@gmail.com>

--- a/js/src/moz.build
+++ b/js/src/moz.build
@@ -785,7 +785,7 @@ if CONFIG['JS_HAS_CTYPES']:
 if CONFIG['CC_TYPE'] in ('clang', 'gcc'):
     # Also disable strict-aliasing for GCC compiler, that is enabled by default
     # starting with version 7.1, see Bug 1363009
-    CXXFLAGS += ['-Wno-shadow', '-Werror=format', '-fno-strict-aliasing']
+    CXXFLAGS += ['-Wno-shadow', '-fno-strict-aliasing']
 
 # Suppress warnings in third-party code.
 if CONFIG['CC_TYPE'] in ('clang', 'gcc'):