[Object/ELF] - Fix the offset type used in ELFFile<ELFT>::getEntry().

We use size_t for a file offset what is wrong, because size_t is 32-bit
value on 32-bit platforms.

I was reported that after my 0b511c23
"[llvm-readobj] - Report warnings instead of errors for broken relocations."

The following error is observed on 32-bit Arch Linux:

[100%] Running all regression tests
FAIL: LLVM :: tools/llvm-readobj/ELF/relocation-errors.test (52954 of 54768)
******************** TEST 'LLVM :: tools/llvm-readobj/ELF/relocation-errors.test' FAILED ***

...
llvm-project/llvm/test/tools/llvm-readobj/ELF/relocation-errors.test:9:14:error: LLVM-NEXT: expected string not found in input
# LLVM-NEXT: warning: '[[FILE]]': unable to print relocation 1 in section 3: unable to access section [index 6] data at 0x17e7e7e8b0: offset goes past the end of file
             ^
<stdin>:9:1: note: scanning from here
/llvm-project/build/bin/llvm-readobj: warning: 'llvm-project/build/test/tools/llvm-readobj/ELF/Output/relocation-errors.test.tmp64': unable to print relocation 1 in section 3: unable to access section [index 6] data at 0xe7e7e8b0: offset goes past the end of file

This patch should fix the issue.