Add MSVC $L local label prefix to is_local_label

[punpckhdq]

MSVC uses an $L prefix when naming branch target labels. These can be emitted as static symbols, here is the output of DUMPBIN /ALL on one of my objects:

    Section length    8, #relocs    0, #linenums    0, checksum D0085986, selection    2 (pick any)
201 00000000 SECT93 notype       External     | __real@3fefd70a40000000
202 000000F2 SECT8E notype       Label        | $L10839
203 000000DB SECT8E notype       Label        | $L10837
204 000000BA SECT8E notype       Label        | $L10835
205 00000094 SECT8E notype       Label        | $L10834
206 00000087 SECT8E notype       Label        | $L10833
207 0000007A SECT8E notype       Label        | $L10832
208 0000006D SECT8E notype       Label        | $L10831
209 000001C8 SECT8E notype       Static       | $L12876
20A 000001E8 SECT8E notype       Static       | $L12863
20B 00000000 SECT94 notype       Static       | .text

This requires a prefix match in is_local_label to handle correctly. Without that, the branch targets are shown as separate local functions in objdiff.

The PR fixes this by adding "$L" to LABEL_PREFIXES. With the patch applied, the same function is shown as matching with the base, because its branch target label is now being merged with the rest of the function: