libsframe: fix sframe_find_fre for pltN entries

libsframe: fix sframe_find_fre for pltN entries

To find SFrame stack trace information from an FDE of type
SFRAME_FDE_TYPE_PCMASK, sframe_find_fre () was doing an operation
like,
  (start_ip_offset & 0xff) >= (pc & 0xff), etc.

This is buggy and needs correction.  The mask 0xff should be 0xf (to
work for a pltN entry of size say, 16 bytes).

At this time, the size of the pltN entry is implicitly assumed to be 16
bytes by libsframe.  In next version of the SFrame format, we can encode
this information explicitly in the SFrame FDE.

For now, we should fix the code to at least behave correctly for the
generated code and the generated SFrame stack trace information for the
pltN entries on x86_64.

libsframe/
	* sframe.c (sframe_find_fre): Correct the bitmask used for
	SFrame FDEs of type SFRAME_FDE_TYPE_PCMASK.