sframe: Ignore section padding when converting endianness

sframe: Ignore section padding when converting endianness

The .sframe section may have a trailing padding due to the architecture-
specific default section alignment.  Do not treat this padding as error
when converting between target and host endianness.

This can be observed when building Binutils with SFrame s390x support on
x86-64 for s390x using configure option "--target=s390x-ibm-linux-gnu"
and running the GAS test suite.

While at it reuse the determined SFrame section header size.

libsframe/
	* sframe.c (flip_sframe): Ignore .sframe section padding.  Reuse
	SFrame header size.

Reported-by: Indu Bhagat <indu.bhagat@oracle.com>
Signed-off-by: Jens Remus <jremus@linux.ibm.com>