perlasm/x86_64-xlate.pl: refine symbol recognition in .xdata.

Hexadecimals were erroneously recognized as symbols in .xdata.

(Imported from upstream's b068a9b914887af5cc99895754412582fbb0e10b)

Change-Id: I5d8e8e1969669a8961733802d9f034cf26c45552
Reviewed-on: https://boringssl-review.googlesource.com/c/34704
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

crypto/perlasm/x86_64-xlate.pl

diff --git a/crypto/perlasm/x86_64-xlate.pl b/crypto/perlasm/x86_64-xlate.pl
index 2e65d1d..78832fc 100755
--- a/crypto/perlasm/x86_64-xlate.pl
+++ b/crypto/perlasm/x86_64-xlate.pl
@@ -894,7 +894,7 @@
 							$var=~s/^(0b[0-1]+)/oct($1)/eig;
 							$var=~s/^0x([0-9a-f]+)/0$1h/ig if ($masm);
 							if ($sz eq "D" && ($current_segment=~/.[px]data/ || $dir eq ".rva"))
-							{ $var=~s/([_a-z\$\@][_a-z0-9\$\@]*)/$nasm?"$1 wrt ..imagebase":"imagerel $1"/egi; }
+							{ $var=~s/^([_a-z\$\@][_a-z0-9\$\@]*)/$nasm?"$1 wrt ..imagebase":"imagerel $1"/egi; }
 							$var;
 						    };