llext: copy complete string sections while parsing ELF

This has several advantages:

1. we don't need any hard assumptions about symbol length. The
current hard-coded limit of 32 characters might well be not true.
2. replaces a lot of code for reading those names with a single
call to a 1-line function to calculate string location.
3. eliminates the need to allocate buffers for exported symbol names
by replacing them with a simple pointer assignment.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>