middle-end: Disable section anchors for VAR_DECLs if -fdata-sections
-fdata-sections places data symbols into their own, unique, named sections.
-fsection-anchors create an anchor to access neighboring symbols
within a section.
When both are enabled, a separate section anchor is created for each
symbol, which provides no benefit.
This patch updates the common gating function use_blocks_for_decl_p() to
return false if -fdata-sections is enabled.
gcc/ChangeLog:
* varasm.c (use_blocks_for_decl_p): Don't use section anchors
for VAR_DECLs if -fdata-sections enabled.