semihosting lseek doesn't use whence.

[SidManning]

This addresses a failure in the pico-libc, test-fread-fwrite test.

Per the spec:
SYS_SEEK (0x0A)
Seeks to a specified position in a file using an offset specified from the start of the file. The file is assumed to be a byte array and the offset is given in bytes.

Entry
On entry, r1 contains a pointer to a two-word data block:

word 1
This is a handle for a seekable file object.

word 2
The absolute byte position to search to.

Change-Id: Ie3e7ea07a5623ee178a98484bbf1cdbdd3363a01

[quic-mathbern]

On a second look, I agree with the change, Sid. Only three comments/suggestions:

• Use GDB_SEEK_SET instead of 0 for the whence
• Rename common_semi_ftell_cb to something like common_semi_fs_reposition_cb, so that the name is generic enough for both ftell and seek.
• Reword the commit message to mention that the fix is due to a spec divergence between arm and hexagon for SYS_SEEK. You can quote the SYS_SEEK entry on the hexagon semihosting spec which says: "Returns the newly set offset on success or -1 on error, with r1 set to the associated errno."

[androm3da]

Sorry - I rebased hex-next this weekend so you'll need to rebase your change. Or: just leave it as-is and let me know when it's ready and I'll cherry-pick it.