Skip symbol tables with invalid string table links

[EricRahm]

When encountering a symbol table (SYMTAB or DYNSYM) whose sh_link field
does not point to a valid string table (STRTAB) section, bloaty would
previously throw an error and stop. This caused bloaty to fail on some
binaries, such as U-Boot, which have such sections.

This change modifies bloaty to warn about the invalid section and skip
it, allowing processing to continue. This matches the behavior of other
tools like readelf, which warn but do not fail on such binaries.

A lit test is also included.

Confirmed that bloaty can now read u-boot without throwing an error:

% build/130/bloaty u-boot -d compileunits
    FILE SIZE        VM SIZE
 --------------  --------------
  51.3%  4.43Mi  71.6%   446Ki    [340 Others]
  18.4%  1.59Mi   0.0%       0    [section .rel.debug_info]
  10.3%   913Ki   0.0%       0    [section .rel.debug_loc]
   4.1%   359Ki   0.0%       0    [section .debug_frame]
   2.1%   188Ki   0.0%       0    [section .debug_loc]
   1.6%   140Ki   0.0%       0    [section .rel.text]
   1.1%  98.9Ki   1.8%  11.1Ki    lib/efi_loader/efi_boottime.c
   1.1%  93.8Ki   3.3%  20.5Ki    drivers/net/e1000.c
   1.0%  91.7Ki   0.0%       0    [section .rel.debug_ranges]
   1.0%  90.5Ki   2.4%  14.8Ki    fs/fat/fat_write.c
   0.9%  81.6Ki  13.1%  81.6Ki    [section .rodata]
   0.9%  75.4Ki   1.7%  10.4Ki    fs/ext4/ext4_common.c
   0.8%  71.6Ki   1.4%  9.01Ki    cmd/eficonfig.c
   0.8%  67.4Ki   0.0%       0    [section .rel.debug_frame]
   0.7%  65.8Ki   0.3%  2.11Ki    drivers/core/ofnode.c
   0.7%  64.1Ki   1.2%  7.25Ki    boot/image-fit.c
   0.7%  63.8Ki   0.5%  3.18Ki    boot/fdt_support.c
   0.6%  57.3Ki   1.4%  8.71Ki    drivers/mmc/mmc.c
   0.6%  51.4Ki   0.5%  3.41Ki    drivers/pci/pci-uclass.c
   0.6%  51.2Ki   0.0%       0    [section .bss]
   0.6%  50.0Ki   0.8%  4.71Ki    drivers/usb/host/xhci.c
 100.0%  8.63Mi 100.0%   623Ki    TOTAL

Fixes #130

[EricRahm]

@haberman this is just converting a TRHOW to a WARN when an invalid strtab link is encountered.