Fix $OPENSSL_ia32cap handling.

The comment says that an "0x" prefix indicates a hex value. However we
always passed PRIu64 as the format specifier for |sscanf|, and |sscanf|
isn't documented to handle an 0x prefix expect for "i"-family format
specifiers. With |PRIu64|, |sscanf| reads any leading "0x" as just zero.

Instead, check for "0x" ourselves and use |PRIx64| if found to parse hex
values.

Change-Id: Id5ed7009d30902022e5ee640e8931bf1431dedc0
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/38264
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/cpu-intel.c

diff --git a/crypto/cpu-intel.c b/crypto/cpu-intel.c
index 832e9d6..cc41fc4 100644
--- a/crypto/cpu-intel.c
+++ b/crypto/cpu-intel.c
@@ -123,9 +123,17 @@
 // and |out[1]|. See the comment in |OPENSSL_cpuid_setup| about this.
 static void handle_cpu_env(uint32_t *out, const char *in) {
   const int invert = in[0] == '~';
-  uint64_t v;
+  const int hex = in[invert] == '0' && in[invert+1] == 'x';
 
-  if (!sscanf(in + invert, "%" PRIu64, &v)) {
+  int sscanf_result;
+  uint64_t v;
+  if (hex) {
+    sscanf_result = sscanf(in + invert + 2, "%" PRIx64, &v);
+  } else {
+    sscanf_result = sscanf(in + invert, "%" PRIu64, &v);
+  }
+
+  if (!sscanf_result) {
     return;
   }