rebulid-elftc.sh: adapt it to [r3870]

**Add a pointer to any prior users list discussion.**
Please use the users list https://groups.google.com/forum/#!forum/DynamoRIO-Users for questions or to propose initial feature ideas if you're not sure there's an existing solution. The users list will reach a wider audience of people who may find the information beneficial. If you have already asked on the users list and the consensus was to file a new issue, please include the URL to the discussion thread here.

**Is your feature request related to a problem? Please describe.**

This feature request related to port this script to MinGW.

**Describe the solution you'd like**

With current state it fails even when forced to use bmake.

```
+ [[ bmake -eq pmake ]]
+ rm -f '*/*.so'
+ svn status
+ grep '?'
+ awk '{print $2}'
+ xargs rm -f
+ bmake clean
===> common
rm -f
===> libelf
bmake.exe[1]: "/home/ofry/dynamorio/ext/drsyms/libelftc/windows-lib64/libelf/makefile" line 165: Cannot open /home/ofry/dynamorio/ext/drsyms/libelftc/windows-lib64/libelf/../mk/elftoolchain.lib.mk

bmake.exe[1]: stopped in C:/msys64/home/ofry/dynamorio/ext/drsyms/libelftc/windows-lib64/libelf
bmake.exe[1]: Fatal errors encountered -- cannot continue*** Error code 1

```

Such clean problem doesn't appear at r3870.

Do you have any implementation in mind for this feature?

**Describe alternatives you've considered**
Is there currently a method for implementing this functionality using the existing API and this requested feature will add convenience? Or is there no method for accomplishing the desired task using the current API?

**Additional context**
Add any other context or screenshots about the feature request here.

Yes, it also related to https://sourceforge.net/p/elftoolchain/tickets/581/

But is this bug still in r3870?