[gdb/testsuite] use -Ttext-segment for jit-elf tests

[gdb/testsuite] use -Ttext-segment for jit-elf tests

Removes the need to manually relocate loaded ELF binary by using a fixed
constant as both mmap base address and as a requested first segment
address supplied to the linker.

In future will enable JIT tests with a valid DWARF debug info.  Current
tests still need to compile without a debug info though, because they do
a function name modification.

gdb/testsuite/ChangeLog:

2020-02-18  Mihails Strasuns  <mihails.strasuns@intel.com>

	* lib/jit-elf-helpers.exp: Supply -Ttext-segment linker flag and
	  define LOAD_ADDRESS/LOAD_INCREMENT macros for the compiled binaries.
	* gdb.base/jit-elf-main.c: Use LOAD_ADDRESS/LOAD_INCREMENT to
	  calculate the mmap address.