Fix bit-/byte-offset mismatch in parameter to read_value_memory

Fix bit-/byte-offset mismatch in parameter to read_value_memory

The function read_value_memory accepts a parameter embedded_offset and
expects it to represent the byte offset into the given value.  However,
the only invocation with a possibly non-zero embedded_offset happens in
read_pieced_value, where a bit offset is passed instead.

Adjust the implementation of read_value_memory to meet the caller's
expectation.  This implicitly fixes the invocation in read_pieced_value.

gdb/ChangeLog:

	* valops.c (read_value_memory): Change embedded_offset to
	represent a bit offset instead of a byte offset.
	* value.h (read_value_memory): Adjust comment.